1. Field of the Invention
The present invention relates to a catalyst composition for preparing carbon nanotube and a process for preparing carbon nanotube using the same. More particularly, this invention relates to a process for preparing carbon nanotube by the chemical vapor deposition method through the decomposition of lower saturated or unsaturated hydrocarbons using a multi-component metal catalyst composition containing active metal catalyst from Co, V, Al and inactive porous support. Further, the present invention affords the carbon nanotube having 5˜30 nm of diameter and 100˜10,000 of aspect ratio in a high catalytic yield.
2. Description of Prior Art
Carbon nanotube has a hexagonal honey comb shape in which one carbon atom is bonded with 3 adjacent carbon atoms. Further, the graphite plane is rolled in a round shape having a nano size diameter. Specific physical properties are shown according to the size or shape of carbon nanotube. The weight is comparatively low due to the hollow type. Further, the electrical conductivity is as good as that of copper, as well as the thermal conductivity is as good as that of diamond. Of course, the tensile strength is not less than that of iron. According to the rolled shape of carbon nanotubes, it can be classified as single walled nanotube, multi-walled nanotube and rope nanotube.
Such carbon nanotube can be generally manufactured by arc-discharge method, laser vaporization method, plasma enhanced chemical vapor deposition method, thermal chemical vapor deposition method, vapor phase growth method or electrolysis method. Among them, chemical vapor deposition method has been preferably used, because the deposition of carbon nanotube can be made by the direct reaction between hydrocarbon gas and metal catalyst without using the substrate plate. Further, high purity of carbon nanotube can be economically manufactured in a large amount according to chemical vapor deposition method.
In chemical vapor deposition method, the metal catalyst is necessarily required. Among the metals, Ni, Co or Fe has been commonly used. Each particle of metal catalysts can act as seed for the formation of carbon nanotube. Therefore, the metal catalyst has been required to be formed as nano size particle. Of course, many researches for developing metal catalyst have been tried.
As a preparation method of metal catalyst reported until now, following preparation methods have been disclosed. First, the method comprising i) preparing a solution containing catalytic metals and support, ii) co-precipitating the catalyst composition by adjusting pH, temperature and/or amount of ingredients, and iii) heat treating under air or other gas atmosphere has been disclosed. Second, the method by drying or evaporating the suspension containing catalytic metal and fine grain support has been disclosed. Third, the method comprising i) ionizing the metal by mixing catalytic metal salt with cation particle support such as zeolite, and ii) reducing the ionized metal into metal particle by hydrogen or other reducing agent at high temperature has been disclosed. Fourth, the method by calcinating catalytic metal with solid oxide support, such as, magnesia, alumina and/or silica has been disclosed. Finally, the method of calcination for a metal composition has been disclosed where spray drying of a catalytic metal precursor solution has been performed before calcination.
In case of catalytic chemical vapor deposition method, acetylene, methane, ethane, ethylene, butane, butene, butadiene, benzene and/or other hydrocarbon can be disclosed as a carbon source. In general, metal catalyst contains metal, metal oxide or reducible metal components. For example, Fe, Mo, Ni, V, Mn, Sn, Co or Cu has been described as a metal. Even though carbon nanotube can be made by each metal catalyst, it has been reported that a metal catalyst composition has been used to enhance the catalytic yield of carbon nanotube.
The formation of carbon nanotube and the characteristics of the formed carbon nanotube can be varied by the metal catalyst or other production conditions. Metal component of catalyst, the combination of metal components, support material, the interaction between catalytic metal and support material, the kind of gas as carbon source, hydrogen or other gas atmosphere, reaction temperature, reaction time and shape of reactor can influence the formation and characteristics of carbon nanotube. Therefore, the optimization of reaction process has been continuously required.
According to catalytic chemical deposition method, the metal catalytic components are slowly consumed in the process of synthesizing carbon nanotube. This consumption of metal catalytic components is caused by the inactivation of metal components by encapping, where carbon atoms encapsulate metal catalytic particle. Generally, re-activation of inactivated catalytic metal is neither possible, nor economical. In some cases, only few grams of carbon nanotube can be obtained using 1 gram of a metal catalyst composition including metal catalyst and support material. Therefore, the development of a high yield metal catalyst composition and of synthetic conditions has been required in order to produce the carbon nanotube in a commercially available scale.
Following technologies have been reported in patent disclosures and references until now.
The method for preparing carbon nanotube was firstly disclosed by Dr. Iijima in Nippon Electric Company (NEC). For preparing single walled carbon nanotube, carbon arc method by arcing the carbon rod containing metal catalyst has been used (S. Iijima, Nature, 354, 56 (1991)). Recently, HIPCO method (high pressure carbon monoxide method) by Dr. R. E. Smalley as well as methane decomposing method using metal catalyst has been disclosed.
On the other hand, in case of preparing multi-walled carbon nanotube, thermal chemical vapor deposition method using carbon monoxide, acetylene or methane as a carbon source in the presence of transition metal catalyst, such as, Fe, Co or Ni as well as arc-discharge method by arcing carbon rod disclosed by Dr. Iijima has been used. Further, catalytic thermal decomposition method has been also used.
According to U.S. Pat. No. 5,165,909 by Hyperion Catalysis International Inc., a method for producing carbon fibrils which comprises i) calcinating a catalyst composition at about 500° C. under air atmosphere after Fe catalyst is supported by Al2O3, ii) reducing the catalyst composition using hydrogen gas at about 900° C., and iii) preparing carbon fibrils by reacting benzene as a carbon source under hydrogen atmosphere at about 1,000° C. has been disclosed. However, the catalytic yield for preparing carbon fibril is not so good. Further, the process for preparing metal catalyst requires complicate steps of calcination and reduction as well as more than 800° C. of high reaction temperature.
S. Takenaka disclosed a method for preparing carbon nanofiber or carbon nanotube using the catalyst composition impregnating cobalt metal salt into Mg, Al, Si and/or Ti metal oxide, followed by pre-reducing said catalyst composition using hydrogen gas at about 500° C. (J. Phys. Chem. B 2004, 108, 11464-11472). Further, carbon nano materials are prepared using methane as a carbon source. However, the conversion ratio from methane to carbon nano material is less than 10 wt % regarding all 4 kinds of metal oxide catalysts. Therefore, this catalyst composition cannot be applied to a commercial use due to its low efficiency.
According to disclosure of Applied Catalysis A: General 260 (2004) 55˜61 by Z. Konya, carbon nanotube preparation method comprising i) impregnating less than 5 wt % of Ni and V metal salt into zeolite support, ii) calcinating the catalyst composition at 700° C. and iii) flowing 30 ml/min of acetylene as a carbon source with 300 ml/min of nitrogen carrier gas in order to synthesize carbon nanotube has been disclosed. Z. Konya also disclosed an analysis method to investigate catalytic active species using XPS study. However, either the catalytic yield or conversion rate of carbon source has not been disclosed.
In order to solve above problems, such as, low catalytic yield, calcination of catalyst, pre-reducing treatment of catalyst, the flowing hydrogen gas with carbon source and more than 800° C. of high reaction temperature, the inventors of present invention developed a novel catalyst composition and preparation method of carbon nanotube using the same. According to the catalyst composition of present invention, the catalytic yield can be enhanced more than 3 times compared to a formerly developed catalyst composition. Further, carbon nanotube having 5˜30 nm of diameter can be obtained in a short reaction time.